The carrier mobility and the dielectric permittivity are constant throughout the sample.

This is a result of higher carrier mobilities and lower resistive device parasitics.

Both changes reduce the carrier mobility, and hence the transconductance.

From the Hall coefficient, we can obtain the carrier mobility as follows:

Thus doping concentration has great influence on carrier mobility.

In practical terms, a low effective mass leads directly to high carrier mobility, favoring higher speed of transport, and greater current carrying capacity.

Measured carrier mobilities for electrons and holes are shown in Figure 4.

The carrier mobilities are much smaller than that of silicon.

However, the very act of making the channel shorter doesn't decrease carrier mobility nor Ion, as you said.

This is due to the high carrier mobility of InSb.