  Motivated by recent observations of "electron-only" magnetic reconnection,
without an ion-scale sheet or ion outflows, in both the Earth's magnetosheath
and in numerical simulations, we study the formation and reconnection of
electron-scale current sheets at low plasma beta. We first show that ideal
sheets collapse to thicknesses much smaller than the ion scales, by deriving an
appropriate analogue of the Chapman-Kendall collapse solution. Second, we show
that, in practice, reconnection onset happens in these collapsing sheets once
they reach a critical aspect ratio, because the tearing instability then
becomes faster than their collapse timescale. We show that this can happen for
sheet thicknesses larger than the ion scale or at only a few times the electron
scale, depending on plasma parameters and the aspect ratio of the collapsing
structure, thereby unifying the usual picture of ion-coupled reconnection and
the new regime of electron-only reconnection. %and thus the usual picture of
ion-coupled reconnection and the new regime of electron-only reconnection can
be unified. We derive relationships between plasma beta, ion-to-electron
temperature ratio, the aspect ratio, electron outflow velocity, and the final
thickness of the sheets, and thus determine under what circumstances
electron-scale sheets form and reconnect.
