  We use the observed amount of subhaloes and line-of-sight dark matter haloes
in a sample of 11 gravitational lens systems from the Sloan Lens ACS Survey to
constrain the free-streaming properties of the dark matter particles. In
particular, we combine the detection of a small-mass dark matter halo by
Vegetti et al. 2010 with the non-detections by Vegetti et al. 2014 and compare
the derived subhalo and halo mass functions with expectations from cold dark
matter (CDM) and resonantly produced sterile neutrino models. We constrain the
half-mode mass, i.e. the mass scale at which the linear matter power spectrum
is reduced by 50 per cent relatively to the CDM model, to be $\log M_{\rm{hm}}
\left[M_\odot\right] < 12.0$ (equivalent thermal relic mass $m_{\rm th} > 0.3$
keV) at the 2$\sigma$ level. This excludes sterile neutrino models with
neutrino masses $m_{\rm s} < 0.8$ keV at any value of $L_{\rm 6}$. Our
constraints are weaker than currently provided by the number of Milky Way
satellites, observations of the 3.5 keV X-ray line, and the Lyman $\alpha$
forest. However, they are more robust than the former as they are less affected
by baryonic processes. Moreover, unlike the latter, they are not affected by
assumptions on the thermal histories for the intergalactic medium.
Gravitational lens systems with higher data quality and higher source and lens
redshift are required to obtain tighter constraints.
