What is a plausible mechanism for the Haber process?
Do you mean the reaction that forms ammonia? If so, it is the Haber Process.
A plausible mechanism (that I just thought up, actually) for this is:
where each step is a reduction step of nitrogen transferring electrons to grab both hydrogen in each step (one N single bond e- pair is donated to a hydrogen, and one e- pair from the hydrogens is donated to break the single bond and bond to the other, cationic nitrogen).
The accepted mechanism is here, though.