Mogami makes an ultra flex 32awg single wire coaxial construction shielded cable, it's .039" diameter and is about as small as you'll find. There is one center conductor and a shield, so 2 wires, use the center conductor as the signal, and the shield as common. Don't be fooled for that short run small 32awg is adequate. Stuff is pretty cheap, usually wire starts getting more expensive when you get this small.
Does common mean the ground?
If so that cable might just be perfect! thanks for the info, I'm trying to do my first headphone mod ^_^ Gonna try adding both a detachable cord and mic to my HD 280 pro Sennheisers :)
Ill buy this wire and if it doesn't work, I will probably just add dual sided connectors.