  A magnetic flux applied along the axis of a nanotube can counteract the
effect of the tube chirality and dramatically affect its conductance, leading
to a way to determine the chirality of a nanotube. The effect of the applied
flux is strongest in the long tube limit where the conductance is (i) either a
sequence of sharp $4e^{2}/h$ height peaks located at integer (in units of the
flux quantum) values of the flux (for an armchair tube) or (ii) a periodic
sequence of pairs of $2e^{2}/h$ height peaks for a chiral tube, with the
spacing determined by the chirality. In the short tube limit the conductance
takes on the value that gives the universal conductivity of an undoped graphene
sheet, with a small amplitude modulation periodic in the flux.
