  We report conductance measurements on multiwall carbon nanotubes in a
perpendicular magnetic field. A gate electrode with large capacitance is used
to considerably vary the nanotube Fermi level. This enables us to search for
signatures of the unique electronic band structure of the nanotubes in the
regime of diffusive quantum transport. We find an unusual quenching of the
magnetoconductance and the zero bias anomaly in the differential conductance at
certain gate voltages, which can be linked to the onset of
quasi-one-dimensional subbands.
