The space dependence of the effective mass in quantum wires results in the appearance of an additional momentum-dependent potential. If the effective mass is anisotropic (as in silicon or germanium), this effect strongly depends on the transverse mass for a given sub-band. As an example, we consider Si-Ge p-type quantum wires, where the impact ionization by holes should be determined by the impact ionization rates in silicon and not in SiGe.