  Contacting metallic single-walled carbon nanotubes by palladium (Pd) affords
highly reproducible ohmic contacts and allows for detailed elucidation of
ballistic transport in metallic nanotubes. The Pd ohmic contacts are more
reliable than titanium (Ti) previously used for ballistic nanotube devices. In
contrast, Pt contacts appear to give non-ohmic contacts to metallic nanotubes.
For both ohmic and non-ohmic contacts, the length of the nanotube under the
metal contact area is electrically turned off. Transport occurs from metal to
nanotube at the edges of the contacts. Measurements with large numbers of Pd
contacted nanotube samples reveal that the mean free path for defect scattering
in SWNTs grown by chemical vapor deposition can be up to 4 microns. The mean
free paths for acoustic phonon scattering are on the order of 500 nm at room
temperature and >> 4 microns at low temperatures.
