  Understanding the temperature dependence of thermal boundary resistance, or
Kapitza resistance, between liquid helium and sintered metal has posed a
problem in low temperature physics for decades. In the ballistic regime of
superfluid $^{3}$He-B, we find the Kapitza resistance can be described via
scattering of thermal excitations (quasiparticles) with a macroscopic geometric
area, rather than the sintered metal's microscopic area. We estimate that a
quasiparticle needs on the order of 1000 collisions to successfully thermalise
with the sinter. Finally, we find that the Kapitza resistance is approximately
doubled with the addition of two mono-layers of solid $^{4}$He on the sinter
surface, which we attribute to an extra magnetic channel of heat transfer being
closed as the non-magnetic solid $^{4}$He replaces the magnetic solid $^{3}$He.
