  We study theoretically electronic transport through a contact of a quantum
wire with 2D or 3D leads and find that if the contact is not smooth and
adiabatic then the conduction is strongly suppressed below a threshold voltage
$V_T$, while above $V_T$ the dc current $\bar I$ is accompanied by coherent
oscillations of frequency $f=\bar I / e$. The effect is related to
interelectronic repulsion and interaction of dc current with the Friedel
oscillations near a sharp contact. In short conducting channels of length $L <
L_0 \simeq \hbar v_F/eV_T$ and at high temperatures $T > T_0 \simeq eV_T/k_B$
the effect is destroyed by fluctuations.
