  We study the rate of convergence to equilibrium for a collisionless
(Knudsen)gas enclosed in a vessel in dimension $n \in \{2,3\}$. By semigroup
arguments,we prove that in the $L^1$ norm, the polynomial rate of
convergence$\frac{1}{(t+1)^{n-}}$ given by Tsuji et al [2010] and Kuo et
al[2013,2014,2015] can be extended to any $C^2$ domain, with standard
assumptionson the initial data. This is to our knowledge, the first
quantitative result incollisionless kinetic theory in dimension equal to or
larger than 2 relying ondeterministic arguments that does not require any
symmetry of the domain, nor amonokinetic regime. The dependency of the rate
with respect to the initialdistribution is detailed. Our study includes the
case where the temperature atthe boundary varies. The demonstrations are
adapted from a deterministicversion of a subgeometric Harris' theorem recently
established by Ca\~nizo andMischler. We also compare our model with a
free-transport equation withabsorbing boundary.
