Configurational dephasing in a weak-localization experiment

We have determined the dephasing time for the two-dimensional electron gas on the surface of a ${\mathrm{H}}_{2}$ crystal from low-field magnetoresistance measurements. We find that a moderate amount of helium gas above the ${\mathrm{H}}_{2}$ 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 ${\mathrm{H}}_{2}$ surface.