Pmos or Nmos? Which has the biggest contribution to flicker noise?
The answer lies in the mean time between collisions and this is based on mobility.  Whichever device has a higher mobility, \$\mu\$, will have higher collisions because you have a greater probability of have a collision.  This generally means that nFETs have higher collisions and therefore higher flicker noise, but on undoped channels, you will see similar mobilities.  I have 10nm fins on my bench that show higher \$\mu_p\$.  

Due to Brownian motion, you have movement whenever you have heat, and skipping a bunch of physics, you end up with the average net velocity for drift to be \$v_{dn}= -\mu_nE\$ and \$v_{dp}= \mu_pE\$ respectively. The mobility \$\mu_x\$ has the "mean free time between collisions" term of \$\tau_c\$, as \$\mu_{n,p}=\frac{q\tau_c}{2m_{n,p}}\$.

Once you calculate \$\tau_c\$, what this tells you is that for a field, \$E\$, for similar devices you will have more collisions just due to higher mobility.  To actually calculate \$\tau_c\$, you will need to pull out a device physics book and look at the density of states at a temperature under field conditions.  This is one of those things that we just empirically measure.  The math says it's proportional to \$T^{\frac{1}{2}}\$, but on the bench you see \$T^{\frac{3}{2}}\$ as your channel changes.