It really depends who you ask, and what you mean by "contemplate". The Buddha said:
When one sees with wisdom that 'all formations are impermanent', this is the path of purification.
So the actual path is through "seeing" impermanence. One might correct your question to say that self-enlightenment can be achieved through seeing impermanence.
It can also be achieved through seeing suffering, or non-self.
One could also adjust and say that self-enlightenment is achieved through contemplating the impermanent, i.e. the five aggregates, since contemplating them - or observing objectively, rather - is what leads one to see impermanence, suffering, and non-self.
"Bhikkhus, how do you conceive it: is form permanent or impermanent?" — "Impermanent, venerable Sir." — "Now is what is impermanent painful or pleasant?" — "Painful, venerable Sir." — "Now is what is impermanent, what is painful since subject to change, fit to be regarded thus: 'This is mine, this is I, this is my self'"? — "No, venerable sir."
-- SN 22.59 (Nyanamoli, trans)