  We present numerical studies of conduction in graphene nanoribbons with
different types of disorder. We find that even when defect scattering depresses
the conductance to values two orders of magnitude lower than 2e^2/h, equally
spaced conductance plateaus occur at moderately low temperatures due to
enhanced electron backscattering near subband edge energies if bulk vacancies
are present in the ribbon. This work accounts quantitatively for the surprising
conductance quantization observed by Lin et al. [Phys. Rev. B 78, 161409
(2008)] in ribbons with such low conductances.
