  Semiconducting single-walled carbon nanotubes are studied in the diffusive
transport regime. The peak mobility is found to scale with the square of the
nanotube diameter and inversely with temperature. The maximum conductance,
corrected for the contacts, is linear in the diameter and inverse temperature.
These results are in good agreement with theoretical predictions for acoustic
phonon scattering in combination with the unusual band structure of nanotubes.
These measurements set the upper bound for the performance of nanotube
transistors operating in the diffusive regime.
