  Similar to the radiative parton energy loss due to gluon bremsstrahlung,
elastic energy loss of a parton undergoing multiple scattering in a finite
medium is demonstrated to be sensitive to interference effect. The interference
between amplitudes of elastic scattering via a gluon exchange and that of gluon
radiation reduces the effective elastic energy loss in a finite medium and
gives rise to a non-trivial length dependence. The reduction is most
significant for a propagation length $L< 4/\pi T$ in a medium with a
temperature $T$. Though the finite size effect is not significant for the
average parton propagation in the most central heavy-ion collisions, it will
affect the centrality dependence of its effect on jet quenching.
