  Let $\kappa \in \mathbb{N}_+^\ell$ satisfy $\kappa_1 + \dots + \kappa_\ell =
n$ and let $\mathcal{U}_\kappa$ denote the "multislice" of all strings $u$ in
$[\ell]^n$ having exactly $\kappa_i$ coordinates equal to $i$, for all $i \in
[\ell]$. Consider the Markov chain on $\mathcal{U}_\kappa$, where a step is a
random transposition of two coordinates of $u$. We show that the log-Sobolev
constant $\rho_\kappa$ for the chain satisfies $$(\rho_\kappa)^{-1} \leq n
\sum_{i=1}^{\ell} \tfrac{1}{2} \log_2(4n/\kappa_i),$$ which is sharp up to
constants whenever $\ell$ is constant. From this, we derive some consequences
for small-set expansion and isoperimetry in the multislice, including a KKL
Theorem, a Kruskal--Katona Theorem for the multislice, a Friedgut Junta
Theorem, and a Nisan--Szegedy Theorem.
