  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 non-adiabatic
scatterer in the point contact, due to backscattering. Recently, it was shown
[Phys. Rev. Lett. 71, 137 (1993)] that taking many non-adiabatic 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 give 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.
