  I argue that ideal MHD relativistic winds are always limited in practice to
asymptotic 4-velocity $\gamma_\infty \approx \sigma_0^{1/3}$ and asymptotic
magnetization $\sigma \sim \sigma_0^{2/3} \gg 1$, where $\sigma_0$ is the wind
magnetization with respect to the rest energy density, evaluated at the light
cylinder of the rotating, magnetized compact object that drives the flow. This
suggests that the observed low value of he asymptotic $\sigma$ in the
equatorial sectors of the winds driving Pulsar Wind Nebulae and the associated
high values of the asymptotic 4-velocity are a consequence of magentic
dissipation in the wind zone.
