LEVEL BROADENING FOR LOCALIZED-ELECTRONS - CHANGE OF THE MAGNETOCONDUCTANCE SIGN

The distribution of the hopping probabilities in the strong localizati on regime is calculated via the independent-directed-path formalism, a llowing for level broadening of the intermediate site energies of the hopping process. The distribution belongs to the Gaussian unitary ense mble even in the absence of a magnetic field, whereas in the absence o f level broadening it belongs to the orthogonal ensemble. In the prese nce of the field it leads to an orbital magnetoconductance quadratic i n the field, changing from positive to negative as the temperature is lowered and the number of paths increases.