  Constructive interference among coherent waves traveling time-reversed paths
in a random medium gives rise to the enhancement of light scattering observed
in directions close to backscattering. This phenomenon is known as enhanced
backscattering (EBS). According to diffusion theory, the angular width of an
EBS cone is proportional to the ratio of the wavelength of light $\lambda$ to
the transport mean free path length ls* of a random medium. In biological
media, large ls* ~ 0.5-2 mm >> $\lambda$ results in an extremely small (~0.001
$^\degree$) angular width of the EBS cone making the experimental observation
of such narrow peaks difficult. Recently, the feasibility of observing EBS
under low spatial coherence illumination (spatial coherence length Lsc<<ls*)
was demonstrated. Low spatial coherence behaves as a spatial filter rejecting
longer path-lengths and, thus, resulting in more than 100 times increase in the
angular width of low coherence EBS (LEBS) cones. However, conventional
diffusion approximation-based model of EBS has not been able to explain such
dramatic increase in LEBS width. Here we present a photon random walk model of
LEBS using Monte Carlo simulation to elucidate the mechanism accounting for the
unprecedented broadening of LEBS peaks. Typically, the exit angles of the
scattered photons are not considered in modeling EBS in diffusion regime. We
show that small exit angles are highly sensitive to low order scattering, which
is crucial for accurate modeling of LEBS. Our results show that the predictions
of the model are in excellent agreement with experimental data.
