  Assume that $(X,d,\mu)$ is a metric space endowed with a non-negative Borel
measure $\mu$ satisfying the doubling condition and the additional condition
that $\mu(B(x,r))\gtrsim r^n$ for any $x\in X, \,r>0$ and some $n\geq1$. Let
$L$ be a non-negative self-adjoint operator on $L^2(X,\mu)$. We assume that
$e^{-tL}$ satisfies a Gaussian upper bound and the Schr\"odinger operator
$e^{itL}$ satisfies an $L^1\to L^\infty$ decay estimate of the form
\begin{equation*} \|e^{itL}\|_{L^1\to L^\infty} \lesssim |t|^{-\frac{n}{2}}.
\end{equation*}
  Then for a general class of dispersive semigroup $e^{it\phi(L)}$, where
$\phi: \mathbb{R}^+ \to \mathbb{R}$ is smooth, we establish a similar $L^1\to
L^\infty$ decay estimate by a suitable subordination formula connecting it with
the Schr\"odinger operator $e^{itL}$. As applications, we derive new Strichartz
estimates for several dispersive equations related to Hermite operators,
twisted Laplacians and Laguerre operators.
