  The electronic properties of carbon nanotubes are investigated in the
presence of disorder and a magnetic field parallel or perpendicular to the
nanotube axis. In the parallel field geometry, the $\phi_{0}(=hc/e)$-periodic
metal-insulator transition (MIT) induced in metallic or semiconducting
nanotubes is shown to be related to a chirality-dependent shifting of the
energy of the van Hove singularities (VHSs). The effect of disorder on this
magnetic field-related mechanism is considered with a discussion of mean free
paths, localization lengths and magnetic dephasing rate in the context of
recent experiments.
