MOSFET as a VCCS or an variable resistance
I'm pretty sure they are the same as it's a voltage controlled current source, but I've never seen that exact description.  A current constraining source would be a resistor.  Usually, as MOSFET as a resistor is an ohmic device, so I pushed through the math as if the device was exclusively a saturated MOSFET resistor (ie: mobility controlled device) and you get the following:
$$R_{ON}=res\left(\frac{L}{t_{inv}W}\right)=\left(\frac{1}{n_{inv}q\mu_n}\right)\left(\frac{L}{t_{inv}W}\right)$$
This then becomes
$$R_{ON}=\frac{1}{\mu_n C_{ox}\frac{W}{L}\left(V_{gs}-V_{th}\right)}$$

because \$ q n_{inv} \mu_n =Q_{inv}=C_{ox} \left(V_{gs}-V_{th}\right)\$
There's your zeroth order analysis.