  Strong electric field annihilation by particle-antiparticle pair creation,
also known as the Schwinger effect, is a non-perturbative prediction of quantum
electrodynamics. Its experimental demonstration remains elusive, as threshold
electric fields are extremely strong and beyond current reach. Here, we propose
a mesoscopic variant of the Schwinger effect in graphene, which hosts Dirac
fermions with an approximate electron-hole symmetry. Using transport
measurements, we report on universal 1d-Schwinger conductance at the pinchoff
of ballistic graphene transistors. Strong pinchoff electric fields are
concentrated within approximately 1 $\mu$m of the transistor's drain, and
induce Schwinger electron-hole pair creation at saturation. This effect
precedes a collective instability toward an ohmic Zener regime, which is
rejected at twice the pinchoff voltage in long devices. These observations
advance our understanding of current saturation limits in ballistic graphene
and provide a direction for further quantum electrodynamic experiments in the
laboratory.
