  We re-visit the issue of plasmon damping due to electron-electron
interaction. The plasmon linewidth can related to the imaginary part of the
charge susceptibility or, equivalently, to the real part of the optical
conductivity, $\mathrm{Re}\sigma(q,\omega)$. Approaching the problem first via
a standard semi-classical Boltzmann equation, we show that
$\mathrm{Re}\sigma(q,\omega)$ of two-dimensional (2D) electron gas scales as
$q^2T^2/\omega^4$ for $\omega\ll T$, which agrees with the results of Refs. [1]
and [2] but disagrees with that of Ref. [3], according to which
$\mathrm{Re}\sigma(q,\omega) \propto q^2T^2/\omega^2$. To resolve this
disagreement, we re-derive $\mathrm{Re}\sigma(q,\omega)$ using the original
method of Ref. {mishchenko:2004} for an arbitrary ratio $\omega/T$ and show
that, while the last term is, indeed, present, it is subleading to the
$q^2T^2/\omega^4$ term. We give a physical interpretation of both leading and
subleading contributions in terms of the shear and bulk viscosities of an
electron liquid, respectively. We also calculate $\mathrm{Re}\sigma(q,\omega)$
for a three-dimensional (3D) electron gas and doped monolayer graphene. We find
that, with all other parameters being equal, finite temperature has the
strongest effect on the plasmon linewidth in graphene, where it scales as
$T^4\ln T$ for $\omega\ll T$.
