Observation of Configurational Dephasing in a Weak Localization Experiment.

We have determined the dephasing time for the 2D electron gas on the surface of H sub 2 crystal from low-field magnetoresistance measurements. We find that a moderate amount of helium gas above the H sub 2 surface causes large increases in resistivity, but unexpectedly small negative magnetoresistance effects. We attribute this to a suppression of weak localization as a result of the dephasing effect of the thermal motion of helium atoms normal to the H sub 2 surface.