  We study the differential conductance in the Kondo regime of a quantum dot
coupled to multiple leads. When the bias is applied symmetrically on two of the
leads ($V$ and $-V$, as usual in experiments), while the others are grounded,
the conductance through the biased leads always shows the expected enhancement
at {\it zero} bias. However, under asymmetrically applied bias ($V$ and
$\lambda V$, with $\lambda>0$), a suppression - dip - appears in the
differential conductance if the asymmetry coefficient $\lambda$ is beyond a
given threshold $\lambda_0= \sqrt[3]{1+r}$ determined by the ratio $r$ of the
dot-leads couplings. This is a recipe to determine experimentally this ratio
which is important for the quantum-dot devices. This finding is a direct result
of the Keldysh transport formalism. For the illustration we use a many-lead
Anderson Hamiltonian, the Green functions being calculated in the Lacroix
approximation, which is generalized to the case of nonequilibrium.
