  We study the weak localization effect in quantum transport through a clean
ballistic cavity with regular classical dynamics. We address the question which
paths account for the suppression of conductance through a system where
disorder and chaos are absent. By exploiting both quantum and semiclassical
methods, we unambiguously identify paths that are diffractively backscattered
into the cavity (when approaching the lead mouths from the cavity interior) to
play a key role. Diffractive scattering couples transmitted and reflected paths
and is thus essential to reproduce the weak-localization peak in reflection and
the corresponding anti-peak in transmission. A comparison of semiclassical
calculations featuring these diffractive paths yields good agreement with full
quantum calculations and experimental data. Our theory provides system-specific
predictions for the quantum regime of few open lead modes and can be expected
to be relevant also for mixed as well as chaotic systems.
