  We study numerically the effects of edge and bulk disorder on the conductance
of graphene nanoribbons. We compute the conductance suppression due to
localization induced by edge scattering. We find that even for weak edge
roughness, conductance steps are suppressed and transport gaps appear. These
gaps are approximately inversely proportional to the nanoribbon width. On/off
conductance ratios grow exponentially with the nanoribbon length. Our results
impose severe limitations to the use of graphene in ballistic nanowires.
