  Superconductivity derives its most salient features from the coherence of its
macroscopic wave function. The associated physical phenomena have now moved
from exotic subjects to fundamental building blocks for quantum circuits such
as qubits or single photonic modes. Here, we theoretically find that the AC
Josephson effect---which transforms a DC voltage $V_b$ into an oscillating
signal $cos(2eV_b t/ \hbar)$---has a mesoscopic counterpart in normal
conductors. We show that on applying a DC voltage $V_b$ to an electronic
interferometer, there exists a universal transient regime where the current
oscillates at frequency $eV_b/h$. This effect is not limited by a
superconducting gap and could, in principle, be used to produce tunable AC
signals in the elusive $0.1-10$ THz "terahertz gap".
