We report on a numerical study of quantum diffusion over micron lengths in
defect-free multi-walled nanotubes. The intershell coupling allows the electron
spreading over several shells, and when their periodicities along the nanotube
axis are incommensurate, which is likely in real materials, the electronic
propagation is shown to be non ballistic. This results in magnetotransport
properties which are exceptional for a disorder free system, and provides a new
scenario to understand the experiments (A. Bachtold et al. Nature 397, 673
(1999)).Comment: 4 page