  Multiple scattering of waves presents challenges for imaging complex media
but offers potential for their characterization. Its onset is actually governed
by the scattering mean free path $\ell_s$ that provides crucial information on
the medium micro-architecture. Here, we introduce a reflection matrix method
designed to estimate this parameter from the time decay of the single
scattering rate. Our method is first validated by means of an ultrasound
experiment on a tissue-mimicking phantom. An in-vivo measurement of $\ell_s$ is
then performed in the human liver. This study thus opens important perspectives
for quantitative imaging whether it be for non-destructive testing or
biomedical applications.
