Once a gas is ionized to a plasma it conducts very well. Above a few thousand degrees the atoms of any molecular gas are fully dissociated, and ionized. Adding anything like oxygen into the mix would just radiate heat away and pollute the fusion reaction. (not to mention that the oxygen would react with everything as it cooled at the end of the shot)

One difference from most normal materials though is that plasmas become better conductors the hotter they get (conductivity rises as T^(3/2) ). So at the 100,000K of a filament the conductivity is similar to a metal like copper. As it gets hotter the conductivity is essentially infinite (ie it is not the limiting factor in the circuit).