  The violation of Wiedemann-Franz (WF) law has been widely discussed in
quantum transport experiments as an indication of deviation from Fermi-liquid
behavior. The conventional form of WF law is only concerned with the
transmission function at Fermi-level which, however, vanishes in many practical
situations. We reinvestigate the WF law in noninteracting quantum systems with
vanishing zero energy transmission and report a universal number $21/5$ as an
upper bound of Lorenz ratio $\mathscr{R}$ in weakly energy-dependent scattering
theory. We provide different experimental realizations for the observation of
$\mathscr{R}=21/5$ namely the transport setups with graphene, the multi-level
quantum dot and double quantum dot. The reported universal Lorenz ratio paves
an efficient way of experimentally obtaining the information about the
associated quantum interferences in the system. Our work also provides enough
evidence which concludes that the violation of WF law does not necessarily
imply the non-Fermi-liquid nature of underlying transport processes; equally,
the Fermi-liquid transport characteristics cannot be concluded by an observed
validation of WF law.
