Values of $\frac{2}{\langle N_{\rm part}\rangle}\langle$d$N_{\rm ch}$d$\eta\rangle$ for central Pb-Pb and Au-Au collisions (see text) as a function of $\snn$. Measurements for inelastic pp and ${\rm p}\bar{\rm p}$ collisions as a function of $\sqrt{s}$ are also shown along with those from non-single diffractive p-A and d-A collisions. The $s$-dependencies of the AA and pp (${\rm p}\bar{\rm p}$) collision data are well described by the functions $s_{\rm NN}^{0.155}$ (solid line) and $s_{\rm NN}^{0.103}$ (dashed line), respectively The shaded bands show the uncertainties on the extracted power-law dependencies. The central Pb-Pb measurements from CMS and ATLAS at 2.76 TeV have been shifted horizontally for clarity.