  We study the dynamical properties of massive quiescent galaxies at $1.4 < z <
2.1$ using deep Hubble Space Telescope WFC3/F160W imaging and a combination of
literature stellar velocity dispersion measurements and new near-infrared
spectra obtained using KMOS on the ESO VLT. We use these data to show that the
typical dynamical-to-stellar mass ratio has increased by $\sim$0.2 dex from $z
= 2$ to the present day, and investigate this evolution in the context of
possible changes in the stellar initial mass function (IMF) and/or fraction of
dark matter contained within the galaxy effective radius, $f_\mathrm{DM}$.
Comparing our high-redshift sample to their likely descendants at low-redshift,
we find that $f_\mathrm{DM}$ has increased by a factor of more than 4 since $z
\approx 1.8$, from $f_\mathrm{DM}$ = $6.6\pm1.0$% to $\sim$24%. The observed
increase appears robust to changes in the methods used to estimate dynamical
masses or match progenitors and descendants. We quantify possible variation of
the stellar IMF through the offset parameter $\alpha$, defined as the ratio of
dynamical mass in stars to the stellar mass estimated using a Chabrier IMF. We
demonstrate that the correlation between stellar velocity dispersion and
$\alpha$ reported among quiescent galaxies at low-redshift is already in place
at $z = 2$, and argue that subsequent evolution through (mostly minor) merging
should act to preserve this relation while contributing significantly to
galaxies overall growth in size and stellar mass.
