with preserved TRS are described by GOE [@Sto99]. In this ensemble the joint probability distribution, $P(\{E_i\})$, of the levels (the eigenvalues of $H$) is induced by a Gaussian distribution of the random real symmetric $H$ with zero mean. The exact expression for $P(\{E_i\})$ is well known to have the following form: $$\label{jpd}
 P(\{E_i\})\propto \prod_{n>m} |E_n-E_m|\exp \Big ({-\frac{N\pi^2}{8}\sum_n E_n^2} \Big).$$ Here, we have chosen the variance of $H$ such that it yields the mean level spacing $\Delta=1/N$ at the spectrum center, $E=0$.

The energy levels, as defined by Eq. (\[jpd\]), exhibit a linear level repulsion. As a result, the energy spectrum displays spectral rigidity which restrains the spectral fluctuations around the mean. This important feature can approximately be taken into account within the so-called picket-fence model of equidistantly spaced levels [@Mold67]. The usefulness of this model is in its simplicity that allows one to treat various resonance phenomena analytically, see, e.g., Refs. [@Mold68; @Sok97; @Jun99]. Here, we employ this model to single out a contribution to $q_n^2$ due to fluctuations of the resonance widths.

As concerns the coupling amplitudes, the results are known to be model independent on statistical assumptions on $V_n^c$ as long as the number of open channels is 