Effects of spin-orbit scattering on hopping magnetoconductivity

The low-field magnetoresistance of Au-doped indium oxide films exhibits a pronounced positive component that persists into the insulating regime. Above a certain degree of disorder, however, only negative magnetoresistance is observed similar to that of the undoped material. For weakly localized samples, it can be shown that the observed magnetoresistance may be accounted for by backscattering in the presence of moderately strong spin-orbit scattering (presumably, off the Au impurities). It is argued that backscattering is still important for samples that are fairly deep in the variable-range-hopping regime, albeit in an indirect way. The apparent insensitivity of the magnetoresistance to spin-orbit scattering in the limit of strong disorder, on the other hand, indicates that neglecting returning loops may be justified at that limit.