Determine the least possible value of $f(1998)$, where $f:\mathbb{N} \to \mathbb{N}$ is a function such that for all $m,n \in \mathbb{N}$,  \[ f\left( n^{2}f(m)\right) = m\left( f(n)\right)^{2}. \]

Let's think step by step and output the final answer within \boxed{}.
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