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

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... 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.