Quantum Transport in a Multiwalled Carbon Nanotube

We report on electrical resistance measurements of an individual carbon nanotube down to a temperature $T=20\mathrm{}\mathrm{mK}$. The conductance exhibits a $\ln T$ dependence and saturates at low temperature. A magnetic field applied perpendicular to the tube axis increases the conductance and produces aperiodic fluctuations. The data find a global and coherent interpretation in terms of two-dimensional weak localization and universal conductance fluctuations in mesoscopic conductors. The dimensionality of the electronic system is discussed in terms of the peculiar structure of carbon nanotubes.