  We study dc and ac transport along armchair graphene nanoribbons using the
${\bf k\cdot p}$ spectrum and eigenfunctions and general linear-response
expressions for the conductivities. Then we contrast the results with those for
transport along ordinary waveguides. In all cases we assess the influence of
elastic scattering by impurities, describe it quantitatively with a Drude-type
contribution to the current previously not reported, and evaluate the
corresponding relaxation time for long- and short-range impurity potentials. We
show that this contribution dominates the response at very low frequencies. In
both cases the conductivities increase with the electron density and show cusps
when new subbands start being occupied. As functions of the frequency the
conductivities in armchair graphene nanoribbons exhibit a much richer peak
structure than in ordinary waveguides: in the former intraband and interband
transitions are allowed whereas in the latter only the intraband ones occur.
This difference can be traced to that between the corresponding spectra and
eigenfunctions.
