  We present an analysis of the clustering of galaxies as a function of their
stellar mass at 1 < z < 2 using data from the NEWFIRM Medium Band Survey
(NMBS). The precise photometric redshifts and stellar masses that the NMBS
produces allows us to define a series of mass limited samples of galaxies more
massive than 0.7, 1 and 3x10^10 Msun in redshift intervals centered on z = 1.1,
1.5 and 1.9 respectively. In each redshift interval we show that there exists a
strong dependence of clustering strength on the stellar mass limit of the
sample, with more massive galaxies showing a higher clustering amplitude on all
scales. We further interpret our clustering measurements in the LCDM
cosmological context using the halo model of galaxy clustering. We show that
the typical halo mass of central and satellite galaxies increases with stellar
mass, whereas the satellite fraction decreases with stellar mass, qualitatively
the same as is seen at z < 1. We see little evidence of any redshift dependence
in the stellar mass-to-halo mass relationship over our narrow redshift range.
However, when we compare with similar measurements at z~0, we see clear
evidence for a change in this relation. If we assume a universal baryon
fraction, the ratio of stellar mass to halo mass reveals the fraction of
baryons that have been converted to stars. We see that the peak in this star
formation efficiency for central galaxies shifts to higher halo masses at
higher redshift, moving from ~7x10^11 Msun at z~0 to ~3x10^12 Msun at z~1.5,
revealing evidence of `halo downsizing'. Finally we show that for highly biased
galaxy populations at z > 1 there may be a discrepancy between the measured
space density and clustering and that predicted by the halo model. This could
imply that there is a problem with one or more ingredients of the halo model at
these redshifts, for instance the halo bias relation or the halo profile.
