  Heat flux suppression in collisionless plasmas for a large range of plasma
$\beta$ is explored using two-dimensional particle-in-cell simulations with a
strong, sustained thermal gradient. We find that a transition takes place
between whistler-dominated (high-$\beta$) and double-layer-dominated
(low-$\beta$) heat flux suppression. Whistlers saturate at small amplitude in
the low beta limit and are unable to effectively suppress the heat flux.
Electrostatic double layers suppress the heat flux to a mostly constant factor
of the free streaming value once this transition happens. The double layer
physics is an example of ion-electron coupling and occurs on a scale of roughly
the electron Debye length. The scaling of ion heating associated with the
various heat flux driven instabilities is explored over the full range of
$\beta$ explored. The range of plasma-$\beta$s studied in this work makes it
relevant to the dynamics of a large variety of astrophysical plasmas, including
the intracluster medium of galaxy clusters, hot accretion flows, stellar and
accretion disk coronae, and the solar wind.
