  The average power spectrum of a pulse reflected by a disordered medium
embedded in an N-mode waveguide decays in time with a power law t^(-p). We show
that the exponent p increases from 3/2 to 2 after N^2 scattering times, due to
the onset of localisation. We compare two methods to arrive at this result. The
first method involves the analytic continuation to imaginary absorption rate of
a static scattering problem. The second method involves the solution of a
Fokker-Planck equation for the frequency dependent reflection matrix, by means
of a mapping onto a problem in non-Hermitian quantum mechanics.
