Numerical results of electrical conductance and magnetoresistance of ferromagnetic tunnel junctions and semiconductor/semimetal junctions are presented, putting an emphasis on effects of disorder on the conductance. It is shown that the disorder strongly affects the tunnelling conductance and magnetoresistance. It is discussed how the interfacial resonant states appearing in semiconductor/semimetal junctions influence the conductance. Numerical simulation of diffusive conductivity of a two-dimensional electron gas with spin-orbit interaction is also performed, and the results are compared with the analytical result.