Frequency domain delay and correlation

Hi, i have a signal S and i like to delay it in frequency domain. This is a little example of the code:

    s = wgn(1,100,1);                   %White Gaussian Noise
    sfft = fft(s)
    sfft_shift = sfft.*exp(-i*2*pi*f*delay)
    s_delay = ifft(sfft_shift)

i have some question:

1) delay should be express in second (continous) or in samples ? or it doesn't matter?

2) when i calculate the autocorrelation of the signal s the coefficient is 1. It is perfectly correlate. But when i calculate the crosscorrelation beetween s and s_delay with xcorr(s,s_delay,'coeff'), the coefficient is not 1 but it seems that the two signal s and s_delay are not completely correlate why?

Thanks Paolo