Magnetoresistance and differential conductance in multiwalled carbon nanotubes

We have measured both the magnetoresistance and differential conductance of multiwalled carbon nanotubes as a function of magnetic field perpendicular to the tube axis. The measured differential conductance showed a large depletion of the density of states near the Fermi level and its magnitude was changed with a magnetic field. It was found that the aperiodic fluctuations and negative magnetoresistance mainly originate from the change of density of the states near the Fermi level with the magnetic field, rather than a quantum interference effect. At a particular magnetic field, the true metallic conduction along the outermost shell was observed, and the conductance approached the theoretical value, ${2G}_0{=4e}^2/h,$ as the temperature was lowered.