  We report measurements of the spatial dependence of the electron energy
distribution in carbon nanotubes, from which electron interactions are
determined. Using nonequilibrium tunneling spectroscopy with multiple
superconducting probes, we characterize electron transport as ballistic or
diffusive, and interactions as elastic or inelastic. We find that transport in
nanotubes is generally diffusive, caused by elastic scattering from a few
defects. However, local inelastic scattering can be tuned `on' or `off' with a
gate voltage.
