  The dynamics of weakly magnetized collisionless plasmas in the presence of an
imposed temperature gradient along an ambient magnetic field is explored with
particle-in-cell simulations and modeling. Two thermal reservoirs at different
temperatures drive an electron heat flux that destabilizes off-angle
whistler-type modes. The whistlers grow to large amplitude, $\delta B / B_{0}
\simeq 1$, and resonantly scatter the electrons, significantly reducing the
heat flux. A surprise is that the resulting steady state heat flux is largely
independent of the thermal gradient. The rate of thermal conduction is instead
controlled by the finite propagation speed of the whistlers, which act as
mobile scattering centers that convect the thermal energy of the hot reservoir.
The results are relevant to thermal transport in high $\beta$ astrophysical
plasmas such as hot accretion flows and the intracluster medium of galaxy
clusters.
