Similarity of Wigner's delay time to the virial theorem for scattering by a central field

The quantum-mechanical virial theorem for scattering of a particle by a central force is expressed in terms of Wigner's definition of time delay, $\tau =\frac{2\hslash d\eta }{\mathrm{dE}}$. By using semiclassical approximations, the quantum-mechanical virial is transformed into the classical virial and $\tau$ becomes exactly the classical delay time.