  We investigate the energy exchanges along an electronic quantum channel
realized in the integer quantum Hall regime at filling factor $\nu_L=2$. One of
the two edge channels is driven out-of-equilibrium and the resulting electronic
energy distribution is measured in the outer channel, after several propagation
lengths $0.8~\mu$m$\leq L\leq30~\mu$m. Whereas there are no discernable energy
transfers toward thermalized states, we find efficient energy redistribution
between the two channels without particle exchanges. At long distances
$L\geq10~\mu$m, the measured energy distribution is a hot Fermi function whose
temperature is lower than expected for two interacting channels, which suggests
the contribution of extra degrees of freedom. The observed short energy
relaxation length challenges the usual description of quantum Hall excitations
as quasiparticles localized in one edge channel.
