Let \( m \) and \( n \) be the coefficients of seventh and thirteenth terms respectively in the expansion of \( \left( \dfrac{1}{3} x^{\dfrac{1}{3}} + \dfrac{1}{2 x^{\dfrac{2}{3}}} \right)^{18} \). Then \( \left( \dfrac{n}{m} \right)^\dfrac{1}{3} \) is:
