Conductance quantization in a periodically modulated quantum channel: Backscattering and mode mixing

It is known that the conductance of a quantum point contact is quantized in units of ${2e}^2/h$ and this quantization is destroyed by a nonadiabatic scatterer in the point contact, due to backscattering. Recently, it was shown [Phys. Rev. Lett. 71, 137 (1993)] that, taking many nonadiabatic scatterers periodically in a quantum channel, the quantization can be recovered. We study this conductance quantization of a periodic system in the presence of a strong defect. A periodic arrangement of double stubs gives remarkable quantization of conductance. A periodic arrangement of double constrictions also gives a very good quantization only when the separation between the constrictions is small. We conclude that conductance quantization of a periodically modulated channel is robust.