  Recently, Nagel and Stein studied the $\square_b$-heat equation, where
$\square_b$ is the Kohn Laplacian on the boundary of a weakly-pseudoconvex
domain of finite type in $\C^2$. They showed that the Schwartz kernel of
$e^{-t\square_b}$ satisfies good "off-diagonal" estimates, while that of
$e^{-t\square_b}-\pi$ satisfies good "on-diagonal" estimates, where $\pi$ is
the Szeg\"o projection. We offer a simple proof of these results, which easily
generalizes to other, similar situations. Our methods involve adapting the
well-known relationship between the heat equation and the finite propagation
speed of the wave equation to this situation. In addition, we apply these
methods to study multipliers of the form $m\l(\square_b\r)$. In particular, we
show that $m\l(\square_b\r)$ is an NIS operator, where $m$ satisfies an
appropriate Mihlin-H\"ormander condition.
