  We report on ALMA continuum observations of the black hole X-ray binary
A0620-00, at an X-ray luminosity nine orders of magnitude sub-Eddington. The
system was significantly detected at 98 GHz (at $44 \pm 7~\mu{\rm Jy}$) and
only marginally at 233 GHz ($20 \pm 8~\mu{\rm Jy}$), about 40 days later. These
results suggest either an optically thin sub-mm synchrotron spectrum, or highly
variable sub-mm jet emission on month timescales. Although the latter appears
more likely, we note that, at the time of the ALMA observations, A0620-00 was
in a somewhat less active optical-IR state than during all published
multi-wavelength campaigns when a flat-spectrum, partially self-absorbed jet
has been suggested to extend from the radio to the mid-IR regime. Either
interpretation is viable in the context of an internal shock model, where the
jet's spectral shape and variability are set by the power density spectrum of
the shells' Lorentz factor fluctuations. While strictly simultaneous
radio-mm-IR observations are necessary to draw definitive conclusions for
A0620-00, the data presented here, in combination with recent radio and sub-mm
results from higher luminosity systems, demonstrate that jets from black hole
X-ray binaries exhibit a high level of variability - either in flux density or
intrinsic spectral shape, or both - across a wide spectrum of Eddington ratios.
This is not in contrast with expectations from an internal shock model, where
lower jet power systems can be expected to exhibit larger fractional
variability owing to an overall decrease in synchrotron absorption.
