From what I have studied, primary halides do not undergo this reaction whereas secondary and tertiary halides get converted into alkanes via this reaction.
Since primary haldies do not show this recation, it can't be an $S_N2$ pathway.
This would make $S_N1$ a possibility, but $S_N1$ is mostly accompanied by $E_1$ pathway. This should lead to formation of alkenes too but I haven't read about alkenes being formed in the above reaction.
So what is the proper mechanism for the reaction then?