Study of the electrical conductances of a series of ultrathin Bi films as a function of increasing thickness has revealed behavior which can be correlated with the onset of superconductivity, even in the insulating state. The conductances of these films scale empirically with a single parameter which falls to zero when the films become superconductive. These observations suggest a direct transition between insulating and superconducting behavior. This transition occurs at a normal-state sheet resistance close to the quantum resistance for pairs, R q =h/4e 2 .

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The temperature-dependent electrical conductances of sets of ultrathin amorphous Pd fdms prepared by successive deposition in situ a t low temperatures have been found to scale over a range of conductances encompassing both the strongly and weakly localized regimes. This scaling, as well as the variation of the scaling parameter with the Boltzmann conductance, suggests a unified picture of the insulator-to-metal transition in two dimensions.