  To study transportation of magnetic flux from large to small scales in
protostellar sources, we analyzed the Nobeyama 45-m N2H+ (1-0), JCMT 850 um
polarization, and ALMA C18O (2-1) and 1.3 mm and 0.8 mm (polarized) continuum
data of the Class 0 protostar HH 211. The magnetic field strength in the dense
core on a 0.1 pc scale was estimated with the single-dish line and polarization
data using the Davis-Chandrasekhar-Fermi method, and that in the protostellar
envelope on a 600 au scale was estimated from the force balance between the
gravity and magnetic field tension by analyzing the gas kinematics and magnetic
field structures with the ALMA data. Our analysis suggests that from 0.1 pc to
600 au scales, the magnetic field strength increases from 40-107 uG to 0.3-1.2
mG with a scaling relation between the magnetic field strength and density of
$B \propto \rho^{0.36\pm0.08}$, and the mass-to-flux ratio increases from
1.2-3.7 to 9.1-32.3. The increase in the mass-to-flux ratio could suggest that
the magnetic field is partially decoupled from the neutral matter between 0.1
pc and 600 au scales, and hint at efficient ambipolar diffusion in the
infalling protostellar envelope in HH 211, which is the dominant non-ideal
magnetohydrodynamic effect considering the density on these scales. Thus, our
results could support the scenario of efficient ambipolar diffusion enabling
the formation of the 20 au Keplerian disk in HH 211.
