  We study low frequency noise in a non-Ohmic region of metallic single walled
and multiwalled carbon nanotubes. The generalized relative noise appear to be
independent of applied bias in the power-law regime of the tubes and in
agreement with theoretical predictions. Beyond the power law regime the
suppression of conductance due to scattering with optical phonons is
accompanied by a reduction of relative noise by an order of magnitude. Mobility
fluctuations in the tubes due to optical phonon scattering cause the unexpected
reduction in the relative noise magnitude which is modeled using a modified
mobility fluctuation picture. The findings have important implications for
metallic nanotubes being used as interconnects in nanoelectronic devices.
