Numeric value of the electrons drift velocity in superconductors Somebody knows the numeric value of electrons drift velocity in superconductors? How this value depends from the used superconductor material? What's about the current? Since the electrical resistance tends to zero, how many electrons can pass an area (a layer) that is occupied e.g. by 100*100 atoms?
 A: From:
Carver Mead's "Collective Electrodynamics"
Section
Magnetic Interaction of Steady Currents
1.11  Current in a Wire
Speaking of superconducting currents in a wire:


*

*At long last, we can visualize the current distribution within the a
superconducting wire itself. Because the skin depth is so small, the
surface of the wire appears flat on that scale, and we can use the
solution for a flat surface. The current will be a maximum at the
surface of the wire, and will die off exponentially with distance
into the interior of the wire. We can appreciate the relations
involved by examining a simple example: A 10-cm-diameter loop of 0.1-
mm-diameter wire has an inductance of $4.4*10^{-7} $ henry (p. 193 in
Ref. 40). A persistent current of 1 ampere in this loop produces a
flux of $4.4*10^{-7} $ volt-second, which is $2.1*10^8$ flux quanta.
The electron wave function thus has a total phase accumulation of
$2.1*10^8$ cycles along the length of the wire, corresponding to a wave
vector $k=4.25*10^9 M^{-1} $. Due to the cyclic constraint on the wave
function, this phase accumulation is shared by all electrons in the
wire, whether or not they are carrying current.
In the region where current is flowing, the moving mass of the
electrons contributes to the total phase accumulation. The 1-ampere
current results from a current density of $6.4*10^{10} $ amperes per
square meter flowing in a thin $“skin” \approx \lambda$ thick,
just inside the surface. This current density is the result
of the $10^{28} $ electrons per cubic meter moving with a velocity of 
$v \approx 20$ meters per second. The mass of the electrons moving at this
velocity contributes $mv/ \hbar=1.7*10^5 M^{-1} $ to the total wave
vector of the wave function, which is less than one part in $10^4$ of
that contributed by the vector potential. That small difference,
existing in about 1 part in $10^6$ of the cross-sectional area, is
enough to bring $k$ and $A$ into balance in the interior of the wire.
The  (p. 193 in Ref. 40) is from
Fields and Waves in Communication Electronics 
by Simon Ramo, John R. Whinnery, Theodore Van Duzer
The velocity in this example is about 20 meters per second. The velocity has no dependence on the material as long as it is superconducting.
