Conductivity in random systems. II. Finite-size-system percolation

A simplified model of hopping conductivity in amorphous systems is considered. The nonexponential prefactor of the conductivity is shown to be related to the size dependence of the percolation radius, complementing the well-known picture that the infinite percolation radius determines the leading exponential factor. This leads to an extrapolation formula for the infinite-system percolation radius which involves the power of the nonexponential dependence. Computations on large numbers of small systems are used to determine the radius and the power.