  We consider scaled Brownian motion (sBm), a random process described by a
diffusion equation with explicitly time-dependent diffusion coefficient $D(t) =
D_0 t^{\alpha - 1}$ (Batchelor's equation) which, for $\alpha < 1$, is often
used for fitting experimental data for subdiffusion of unclear genesis. We show
that this process is a close relative of subdiffusive continuous-time random
walks and describes the motion of the center of mass of a cloud of independent
walkers. It shares with subdiffusive CTRW its non-stationary and non-ergodic
properties. The non-ergodicity of sBm does not however go hand in hand with
strong difference between its different realizations: its heterogeneity
("ergodicity breaking") parameter tends to zero for long trajectories.
