  Multiwalled carbon nanotubes are shown to be ballistic conductors at room
temperature, with mean free paths of the order of tens of microns. These
experiments follow and extend the original experiments by Frank et al (Science,
280 1744 1998) including in-situ electron microscopy experiments and a detailed
analysis of the length dependence of the resistance. The per unit length
resistance r < 100 Ohm/micron, indicating free paths l > 65 microns,
unambiguously demonstrate ballistic conduction at room temperature up to
macroscopic distances. The nanotube-metal contact resistances are in the range
0.1-1 kOhm micron. Contact scattering can explain why the measured conductances
are about half the expected theoretical value of 2 G0 . For V>0.1V the
conductance rises linearly (dG/dV~0.3 G0 /V) reflecting the linear increase in
the density-of-states in a metallic nanotube above the energy gap. Increased
resistances (r =2- 10 k Ohm/micron) and anomalous I-V dependences result from
impurities and surfactants on the tubes.Evidence is presented that ballistic
transport occurs in undoped and undamaged tubed for which the top layer is
metallic and the next layer is semiconducting. The diffusive properties of
lithographically contacted multiwalled nanotubes most likely result from
purification and other processing steps that damage and dope the nanotubes
thereby making them structurally and electronically different than the pristine
nanotubes investigated here.
