Spectral asymptotics for the semiclassical Dirichlet to Neumann operator

Let $M$ be a compact Riemannian manifold with smooth boundary, and let $R(\lambda)$ be the Dirichlet-to-Neumann operator at frequency $\lambda$. We obtain a leading asymptotic for the spectral counting function for $\lambda^{-1}R(\lambda)$ in an interval $[a_1, a_2)$ as $\lambda \to \infty$, under the assumption that the measure of periodic billiards on $T^*M$ is zero. The asymptotic takes the form \begin{equation*} N(\lambda; a_1,a_2) = \bigl(\kappa(a_2)-\kappa(a_1)\bigr)\mathsf{vol}'(\partial M) \lambda^{d-1}+o(\lambda^{d-1}), \end{equation*} where $\kappa(a)$ is given explicitly by \begin{equation*} \kappa(a) = \frac{\omega_{d-1}}{(2\pi)^{d-1}} \biggl( -\frac{1}{2\pi} \int_{-1}^1 (1 - \eta^2)^{(d-1)/2} \frac{a}{a^2 + \eta^2} \, d\eta - \frac{1}{4} + H(a) (1+a^2)^{(d-1)/2} \biggr) \end{equation*} with the Heavyside function $H(a)$.