  We investigate the effects of a transverse magnetic field on the transport
properties of carbon nanotubes, making use of a long-wavelength description in
terms of Dirac fermion fields. For values of the magnetic length smaller than
the nanotube radius, we observe that the electronic states organize into
incipient Landau subbands, with a highly degenerate level at zero energy. We
show that only the states in dispersive branches, localized at the flanks of
the nanotube, are able to transport current in the longitudinal direction. This
is at the origin of the quantization of the Hall conductivity, that turns out
to be given by even multiples of 2 e^2 /h. We also analyze the effects of the
electron-electron interaction, showing that the magnetic field induces a
suppression of the electronic correlations, reflected in particular in the
enhancement of the tunneling density of states near the Fermi level.
