  We investigate the conductance of normal-conductor/graphene/normal-conductor
(NGN) junctions for arbitrary on-site potentials in the normal and graphitic
parts of the system. We find that a ballistic NGN junction can display
insulating behavior even when the charge-carrier density in the graphene part
is finite. This effect originates in the different k intervals supporting
propagating modes in graphene and a normal conductor, and persists for moderate
levels of bulk, edge, or interface disorder. The ensuing conductance thresholds
could be used as an electronic tool to map out details of the graphene band
structure in absolute k space.
