  High bias electrical transport characteristics of freely suspended metallic
single-walled carbon nanotubes (SWNTs) are investigated at 250-400K in vacuum
and various gases. Self-heating is exploited to examine the temperature
dependence of phonon transport and optical phonon decay in SWNTs. The acoustic
phonon thermal conductivity of a SWNT follows ~1/T at high temperatures.
Non-equilibrium optical phonon effects in suspended nanotubes decrease as the
ambient temperature increases. Gas molecules assist the relaxation of hot
optical phonons along the tube length and enable enhanced current flow. In
vacuum, high bias breakdown of suspended SWNTs can occur via melting caused by
electrically emitted hot optical phonons at a low acoustic phonon temperature.
