  Magnetars are a class of highly magnetized, slowly rotating neutron stars,
only a small fraction of which exhibit radio emission. We propose that the
coherent radio curvature emission is generated by net charge fluctuations from
a twist-current-carrying bundle (the j-bundle) in the scenario of
magnetar-quake. Two-photon pair production is triggered, which requires a
threshold voltage not too much higher than $10^9$ V in the current-carrying
bundle, and which can be regarded as the `open field lines' of a magnetar.
Continued untwisting of the magnetosphere maintains change fluctuations, and
hence coherent radio emission, in the progressively shrinking j-bundle, which
lasts for years until the radio beam is too small to be detected. The modeled
peak flux of radio emission and the flat spectrum are generally consistent with
the observations. We show that this time-dependent, conal-beam, radiative model
can interpret the variable radio pulsation behaviors and the evolution of the
X-ray hot spot of the radio transient magnetar XTE J1810$-$197 and the high-$B$
pulsar/AXP PSR J1622$-$4950. Radio emission with luminosity of
$\lesssim10^{31}\,{\rm erg\,s^{-1}}$ and high-frequency oscillations are
expected to be detected for a magnetar after an X-ray outburst. Differences of
radio emission between magnetars and ordinary pulsars are discussed.
