How to change the quality of an animated gif via imagick in php

Question

I want to change quality of animated gif image via imagick in php. I wrote the below code :

$source = 'images/b.gif';
$imageDesc = 'images/finally.gif';
$thumb = new Imagick($source); 
$thumb = $thumb->coalesceImages(); 
foreach ($thumb as $frame) 
{ 
    $frame->thumbnailImage(300,300); 
} 

$thumb = $thumb->deconstructImages(); 
$thumb->setImageCompressionQuality(20);
$thumb->writeImages($imageDesc, true);

finally image ($imageDesc) was created but image's quality not be changed!