  A crossover between logarithmic and exponential temperature dependence of the
conductance (weak and strong localization) has been observed in ultrathin films
of metals deposited onto substrates held at liquid helium temperatures. The
resistance at the crossover is well defined by the onset of a nearly linear
dependence of conductance on thickness at fixed temperature in a sequence of in
situ evaporated films. The results of a finite size scaling analysis treating
thickness as a control parameter suggest the existence of a T=0 quantum
critical point which we suggest is a charge, or electron glass melting
transition.
