We report the electrical characteristics of substrate-supported metallic single walled carbon nanotubes at temperatures up to 573 K over a range of bias voltages (Vb) for zero gate voltage in air under atmospheric pressure. Our results show a monotonic increase in resistance with temperature, with an I- Vb characteristic that is linear at high temperature but nonlinear at low temperature. A theory for electrical resistance is applied to the data which shows that the transition to Ohmic behavior at high temperature is the result of optical phonon absorption rather than acoustic phonon scattering. © 2008 American Institute of Physics.