EDIT: won't this ring of foil (or whatever) for the neg contact in the head wear out faster from the twisting than if it DID contact through the threads?
If the contact surface at the top of the battery tube is clean, as well true and square to the axis of rotation, the contact area will be fairly broad, and the battery tube will not turn very far once the surfaces meet (which should minimize wear).
I was concerned about this problem with my Rebel 100 moded Ultrafire B3. The B3 uses the same twisty arrangement, but it's PCB looked like the foil was a bit flimsier than the L0D, so I ran my soldering iron around the rim of the ring contact in one nice even circular pass, and built up a thin even layer of solder around the contact ring. The solder I used is a nice eutectic alloy of 62% Tin, 36% Lead, 2% Silver, which not only melts about 5 degrees cooler than the standard 63% Tin, 37% Lead solder mix, but is also stronger due to the silver content.
This is a big hassle though, because you have to be pretty handy with a soldering iron to pull this off without shorting something out by getting solder where it's not supposed to be. Fortunately, unlike the Ultrafire B3, the ring contact on my Fenix L0D looked pretty solid right out of the factory, so I didn't bother to try to coat it with any more solder, and so far it still looks almost new, even after several months.
I will confess that I did add a thin layer (about a half mm or so) of solder to the center contact dot on the underside of the L0D's driver though. The center contact is a LOT easier to solder, and I figured that the center dot sees a lot more friction.
If you turn the light a quarter turn every time you power cycle it from off to on, then the outer ring only contacts with modest pressure during the last few degrees of travel, but that center pin of the battery is pushing up hard against the center contact, under a strong spring load, during the whole 90 degree cycle.
Depending on the shape of the positive battery contact, the point of wear can be fairly small and concentrated, and I thought I was beginning to see some slight wear, so I figured 'better safe than sorry' and gave it a quick dab of solder.
Not something I would necessarily recommend to everyone though, because there are lot's of L0D's out there going strong after several years without this treatment, and it's not a good idea to risk it if your not real handy with a soldering iron.
Also, on another subject, I should have mentioned before that another thing that I have found to be a bit more critical for one-handed operation with the L0D than with some other lights, is the type of lubrication you use on the O-Ring.
The O-Rings Fenix uses on this light are really good at squeegeeing right trough most lubricants and creating O-Ring 'stiction' issues (creating a lot of turning resistance).
For example silicone grease, which might work ok with another light, is a HORRIBLE situation with the L0D. If anyone out there is using the wrong lubrication, then that alone would explain why you can't use your L0D one handed. With my Jetbeam C-LE, the tough issue was finding a grease that would minimize wear on the soft aluminum threads, but with the Fenix L0D-Q4, it was getting smooth operation and preventing the O-ring from binding.
I finally ended up using Locktite Krytox high performance lubricant grease on my L0D-Q4 to get past the O-Ring sticking issues. I'm not a big fan of Krytox overall, because this stuff is NOT what you want to use on lights with bare aluminum threads (because the PTFE particles will actually cause excessive wear), but on the HAIII anodized threads of my L0D, it seems to be working like a champ.
Krytox costs an arm and a leg though (about 45 bucks for 2oz tube!), so most folks will want to try something else like Nyogel 760G or simple lithium grease. For example, Mobil 1 car grease is a synthetic lithium grease which will work well on both bare aluminum threads, and anodized threads.
With Nyogel 760, Mobil-1 Grease, or simple Lithium Grease, there may be some slight 'stiction' as the O-Ring on the L0D first starts to turn, but once you get it moving, it turns quite easily, making for quite smooth one handed operation. (Some folks may even like the initial 'stiction' effect in the o-ring anyway, because it makes it very unlikely that the L0D will turn on accidentally in your pocket.)