  Explicit expressions of the band spectrum near the neutrality point for
armchair and zigzag graphene ribbons and carbon nanotubes were derived based on
a tight-binding macromolecule model of graphene. The obtained dispersion
relations are used to determine and compare the conductance ladders of these
structures and are beneficial for many other applications. Our results are
different in many respects from previously reported data. This new information
forms a solid reliable basis for evaluation of the potential of graphene
nanoelectronic devices.
