def getR(self, i=5, j=6):
        """ return transport matrix element, indexed by i, j,
        be default, return dispersion value, i.e. getR(5,6) in [m]

        :param i: row index, with initial index of 1
        :param j: col indx, with initial index of 1
        :return: transport matrix element
        """
        if self.refresh is True:
            self.getMatrix()
        return self.transM[i - 1, j - 1]