  Mass dependence of the effective source radii, observed in hadronic $Z^0$
decays by several LEP I experiments, is analyzed in a model which assumes
proportionality between four-momentum of a produced particle and the
four-vector describing its space-time position at the freeze-out. It is shown
that this relation (commonly accepted in description of high-energy collisions)
can explain the data, provided all particles are emitted from a "tube" of $\sim
1$ fm in diameter at a constant proper time $\sim 1.5$ fm.
