We have measured the device characterisics of short and long channel inverted- staggered hydrogenated amorphous silicon thin film transistors (TFTs) with either Mo or Cr source/drain metal after annealing at temperatures from 225 C to 275 C. The TFT deposition temperature at the substrate surface was about 270 C. From the slope of the transfer characteristic an effective mobility is extracted. Devices with Mo source/drain metal exhibit an initial effective mobility increase at short times (within about 30 min), while those with Cr do not. At long times the mobility of all devices decreases. The mobility changes are greatest for short channel length devices because of contact effects. The channel length dependence of the behavior permits a separation of the device behavior into contact and intrinsic mobility components.