Conductance fluctuations near the ballistic-transport regime

The fluctuations in the electric conduction of a short disordered region are studied theoretically. The ensemble-averaged conductance and its fluctuations are calculated analytically in the single-site approximation. For a small number of impurities, the mean square of the fluctuations increases in proportion to the number of impurities, but is independent of the system size; i.e., each impurity contributes independently to the fluctua- tions. Numerical simulations confirm the analytical results. The amplitude of the fluctuations can be larger than the universal values in diffusive conductors when the impurity scattering potential is sufficiently strong. In terms of random matrix theory, the enhancement can be understood by a reduced spectral rigidity of the transmission matrix eigenvalues. @S0163-1829~96!00839-9#