  A pulsar wind is a relativistic outflow dominated by Poynting energy at its
base. Based on the standard ideal magnetohydrodynamic (MHD) model of pulsar
wind nebulae (PWNe) with the ordered magnetic field, the observed slow
expansion $v_{\rm PWN} \ll c$ requires the wind to be dominated by kinetic
energy at the upstream of its termination shock, which conflicts with the
pulsar wind theory ($\sigma$-problem). In this paper, we extend the standard
model of PWNe by phenomenologically taking into account conversion of the
ordered to turbulent magnetic field and dissipation of the turbulent magnetic
field. Disordering of the magnetic structure is inferred from the recent
three-dimensional relativistic ideal MHD simulations, while magnetic
dissipation is a non-ideal MHD effect requiring a finite resistivity. We apply
this model to the Crab Nebula and find that the conversion effect is important
for the flow deceleration, while the dissipation effect is not. Even for
Poynting-dominated pulsar wind, we obtain the Crab Nebula's $v_{\rm PWN}$ by
adopting a finite conversion time-scale of $\sim 0.3$ yr. Magnetic dissipation
primarily affects the synchrotron radiation properties. Any values of the
pulsar wind magnetization $\sigma_{\rm w}$ are allowed within the present model
of the PWN dynamics alone, and even a small termination shock radius of $\ll
0.1$ pc reproduces the observed dynamical features of the Crab Nebula. In order
to establish a high-$\sigma_{\rm w}$ model of PWNe, it is important to extend
the present model by taking into account the broadband spectrum and its spacial
profiles.
