Anderson localization of 2D electrons in dense sup 4 He gas.

We have measured the magnetoconductance of a non- degenerate 2DEG as a function of ambient sup 4 He gas density. At low densities we find that localization effects are suppressed by a configurational dephasing due to the thermal motion of the sup 4 He atoms. At higher densities, the conductivity is observed to decrease exponentially with helium density which we attribute to strong localization of electrons below a threshold energy E sub c > k sub B T. We present a simple estimate of E sub c based on density fluctuations in the gas and find good agreement with experimental observations.