Behavioral inductor (current dependent) in LTspice
The x signifies the derivative, so you have to integrate your expression, first, in order to obtain it correctly.

For your case:

\$f(x)=2*10^{-11}*x^2-3*10^{-9}*x+10^{-6}\$

\$\int{f(x)}{dx}=6.666*10^{-12}*x^3-1.5*10^{-9}*x^2+10^{-6}*x\$

To verify the inductance, you can use a unity ramp current source, which will give a voltage proportional to its inductance:



Notes (in case you want to try it out yourself):


Since your function varies slowly, I chose an interval of 200s to show its variation which, given the great dynamic range difference between the function's values and the time range, LTspice, by default, can show misleading results, so I disabled the waveform compression (imposing a small enough timestep would also have been enough).
There is a sharp rise right at the beginning. That is due to the derivative, I'm afraid it "comes with the territory".