  We investigate the effect of topological defects on the transport properties
of a narrow ballistic ribbon of graphene with zigzag edges. Our results show
that the longitudinal conductance vanishes at several discrete Fermi energies
where the system develops loop orbital electric currents with certain
chirality. The chirality depends on the direction of the applied bias voltage
and the sign of the local curvature created by the topological defects. This
novel quantum blockade phenomenon provides a new way to generate a magnetic
moment by an external electric field, which can prove useful in carbon
electronics.
