  Detailed experimental studies of the conductance of mesoscopic GaAs devices
in the few-mode regime reveal a novel thermal effect: for temperatures up to at
least 10 K the measured gate characteristics, i.e. conductance $G$ versus gate
voltage $V_g$, exhibit a systematic downward shift in gate voltage with
increasing temperature. The effect is 'universal', in the sense that it is
observed in different modulation doped GaAs/GaAlAs heterostructures, in
different device geometries, and using different measurement setups. Our
observations indicate that the effect originates in the surrounding 2D electron
gas and not in the mesoscopic devices themselves.
